Group-III nitride semiconductors, which are compound semiconductors typical of GaN, have a wide band gap, and therefore, they are widely used as materials for light-emitting devices, such as blue, green, and other color LEDs (light-emitting diodes), LDS (laser diodes), and the like, and power devices. In manufacturing semiconductor devices, such as LSIs, and the like, using silicon, and the like, a wafer having a large diameter that is obtained by cutting from a bulk crystal with a large diameter is used, while, for such compound semiconductors, it is difficult to obtain a bulk crystal having a large diameter (for example, 4-inch dia or larger). Therefore, in manufacturing a semiconductor device using such a compound semiconductor, a wafer in which this compound semiconductor is heteroepitaxially grown on a substrate formed of a material dissimilar thereto is generally used. In addition, a p-n junction or a heterojunction which constitutes an LED or an LD can also be obtained by further carrying out an epitaxial growth thereon.
For example, as a material of an epitaxial growth substrate on which a GaN single crystal can be grown, sapphire, SiC, and the like, are known. With these materials, a bulk crystal having a large diameter can be relatively easily obtained, and by selecting the plane orientation therefor as appropriate, a GaN single crystal can be heteroepitaxially grown on a substrate made of such a material, whereby a wafer having a large diameter in which a GaN single crystal is formed can be obtained. However, the substrate made of such a material is expensive, and therefore, the use of a silicon (Si) single crystal substrate, which is less expensive, is being investigated.
In the case of such a heteroepitaxial growth, there exists a difference in lattice constant between the material forming the substrate and the semiconductor layer (for example, a group-III nitride). Due to such difference in lattice constant (the lattice mismatch), there may occur a crystal defect (a dislocation, or the like) or cracks in the semiconductor layer. Patent document 1 discloses a technology for reducing the adverse influence attributable to this lattice mismatch. With the technology, a buffer layer is inserted between the active layer (the layer directly related to the element operation) which is required to be of especially good quality and the Si single crystal substrate, thereby alleviating the lattice mismatch between the active layer material and the Si. Patent document 1 states that, if an AlGaInN multilayer film is formed as a buffer layer on a Si single crystal substrate the surface of which is a (111) plane, a good quality group-III nitride semiconductor layer can be obtained thereon.
Further, in the case of heteroepitaxial growth, a chemical reaction between the material forming the substrate and the semiconductor layer can have an adverse influence on the semiconductor layer grown. The adverse influence resulting from a chemical reaction occurs especially in the edge portion of a wafer, and this influence can extend to the center portion of the semiconductor layer. Patent document 2 discloses a technology for reducing this. With this technology, a protection film made of SiN is formed in the edge portion of the substrate wafer (silicon). If a group-III nitride semiconductor layer is grown thereon, the SiN will become a barrier against such chemical reaction, thereby the chemical reaction will not be caused in the edge portion of the wafer. Thus, by suppressing the chemical reaction in the edge portion, a good quality group-III nitride semiconductor layer can be obtained on the silicon substrate.